Measuring TT Speed

I prefer and feel more confident with the off board Strobe.
They usually cost the price of a Vinyl Album or Two, and don't get a lot of usage once utilised.
A Strobe built in to a TT can offer inaccuracies in showing the Speed,
it is worth reading up on some of the descriptions how this can be happening.

  

the RPM app for cell phones is accurate and a lot easier to use than a strobe. Check it out !

In my phone LG v30+ RPM metter shows 32.83 instead of 33.33
Both my turntables has strobe's and one of the even has custom built. And if I do measure friends machines I am getting the same error always so for me it's usable but it's not measurement tool. It's phone with gyroscope which is not calibrated.

its a bit extreme but the proper way to measure is with a calibrated test tone disk and a frequency counter. You can also send the output to a scope and visually watch the deviations.

jsman,
yes, it's the illuminated number icons that count. If there's drift, and you have a way to adjust platter speed, you can usually get the illuminated numbers to hold steady, plus or minus. I always make speed adjustments with the tonearm lowered and playing the first song of a LP sitting under the strobe disc. It's surprising how much stylus drag there is.
If you don't have a way to adjust speed you can get a close estimate of actual speed and error using this technique posted at the KAB website:

TO EVALUATE DRIFTING SPEED, [COUNT] THE NUMBER OF ICONS THAT MOVE IN AND OUT OF THE SPOTLIGHT IN 60 SECONDS. 

ACTUAL SPEED = 1 + (#ICONS DRIFT PER MIN / 3600) X RPM 

DRIFT IN PERCENT = (#ICONS DRIFT PER MIN / 3600) X 100